Radar observations of long-lost asteroid 1937 UB (Hermes)

Background

Asteroid 1937 UB (Hermes) is a large (~1 km)
object of considerable historical and dynamical interest. It eluded
astronomers for 66 years between its 1937 apparition and 2003
rediscovery.
As of Oct 2003, it was the largest asteroid ever observed to make a
close approach to Earth within less than twice the distance to the
Moon. Hermes is a potentially hazardous asteroid.

The interest in Hermes is not limited to its history or hazard. In
2002 we observed an asteroid, 2002
SY50, that bears a striking resemblance to Hermes in terms of size
and orbital parameters, and it is possible that the two objects are
dynamically related. A close planetary encounter might have tidally
disrupted a large object, leaving Hermes and 2002 SY50 on similar
orbits.

Rediscovery

Hermes was rediscovered on 2003 Oct 15 by Brian
Skiff of Lowell Observatory.
The
rediscovery announcements appeared in
MPEC 2003-T74
and
IAUC circular 8223.
A link with the 1937 observations was rapidly obtained by
Steven Chesley and Paul Chodas (JPL); their work is summarized on
MPEC 2003-U04.
Shortly after the recovery, Andy Rivkin and Rick Binzel (MIT) reported
an S-type classification in IAUC circular
8225.
Our initial radar results are reported in
IAUC circular 8227.
The text of our IAU circular is also available here.

Radar observations

An urgent proposal was submitted to
Arecibo Observatory on Oct 15 (project R1898), and time was allocated
on Oct 17, 19, and 20. Additional observations are planned on Oct
25-26 thanks to the generosity of the aeronomy group at Arecibo.
Time has also been requested at Goldstone, and we will observe Hermes
on Nov 4, 8, 9, 11, 12.

A Doppler spectrum of the radar echo obtained on Oct 17 shows two clearly separated components.

The set of radar images below show the relative motion of the
components over the course of about one hour on Oct 19. Motion is
counter-clockwise. In each panel, the component at positive Doppler
frequencies (right), moves toward the observer, while the component at
negative Doppler frequencies (left), moves away from the observer.
Radar illumination is from the top. The range resolution is 75 m.

Binary Orbit

Measurements of the range and Doppler positions of the primary and
secondary allows us to solve for the orbit of the binary system. The
semi-major axis is about 1.2 km, or about 4 times the radius of the
primary and secondary components. The orbital period is 13.9 hours,
which corresponds exactly to the spin period of both components,
i.e. the system is fully tidally despun to a so-called doubly
synchronous state (like the trans-Neptunian object Pluto-Charon).

Analysis of CW data

Doppler broadening of the Hermes primary and secondary based on CW
data obtained 2003 Oct 18, Nov 04, Nov 08, and Nov 11. Echo widths
were measured at a power level equivalent to three standard deviations
of the noise, were assigned error bars corresponding to the frequency
resolution, and were converted to a common 12.6-cm wavelength. The
solid lines represent the expected Doppler broadening of bodies with
315-m and 280-m radii if their spin axes are perpendicular to our
orbital plane solution. The orbit determination process does not
incorporate any constraints related to the Doppler broadening of the
individual bodies. The good agreement suggests that the orbital
solution is accurate, that the primary and secondary are not
appreciably elongated, and that their obliquities have been driven to
zero by tides.

Radar Astrometry

Range and Doppler astrometry data were reported to the Minor Planet
Center and have been posted on JPL's radar astrometry web site. Here is a statement from Jon Giorgini about the importance of the measurements:

"Using radar, the position and velocity of Hermes has been measured to
within 300 meters and 50 mm/s; much more accurately than with optical
telescopes. Including those measurements in a new orbit solution
suddenly allowed us to accurately predict Hermes motion over many
centuries, from 1561 to 2103.

During those 542 years, it makes 23 close approaches to the Earth,
29 to Venus, 7 to Mars, and 7 to a large asteroid called Vesta. Hermes
closest approach was in the skies over the Earth in 1942, as World War
II was fought underneath, when it passed about 1.8 times further away
than the Moon. However, no one noticed it at the time.

Eventually, the gravitational forces from all these encounters
with the planets blurs out our knowledge of where Hermes will be,
requiring more measurements in the future to make specific longer
predictions. Although technically classified as a "Potentially
Hazardous Asteroid", this only means it has an orbit such that, over
hundreds of thousands or millions of years, Hermes could potentially
come very near the Earth. However, there is no risk now."